As a seal structure of a rotating shaft part in a rotary machine, such as a steam turbine, a gas turbine, a compressor, etc., a so-called labyrinth seal structure, such as those mentioned in, for example, Patent Citation 1 and Patent Citation 2, is widely used.
The labyrinth seal structure is formed of a plurality of fins that project in a ring-shape and that are provided along the axial direction on a rotating shaft or a stationary part opposed thereto and a surface opposed to these fins (opposing surface). Even though the fins are machined to have sharp tips of about 0.2 mm in order to reduce the influence of contact with the opposing surface, since it essentially involves contact between metals, the amount of heat generated by the sliding is great, and a shaft vibration may be caused during heavy contact; therefore, the clearance between the fins and the opposing surface cannot easily be narrowed.
Since the sealing performance, in other words, the performance of a rotary machine, is determined by the number of fins and the clearance between the fins and the opposing surface, it is necessary to narrow the clearance between the fins and the opposing surface in order to improve the performance of the rotary machine. For example, in Patent Citation 1, there is disclosed a structure that employs an Active Clearance Control (ACC) seal, and the clearance is changed by utilizing the differential pressure between a transition period at start-up time and during rated operation. Furthermore, in Patent Citation 2, reduction of the heat generated during contact is attempted by applying an abradable portion, which can be abraded easily, on a contact surface of an opposing surface of a stationary part facing the fins.
A rotary machine is generally designed such that a rotating shaft rotates quietly and constantly in the rated rotational speed range, and there is a speed range (hereinafter this is referred to as a critical speed range) where, soon after start-up, the vibration level of the rotating shaft becomes the maximum while the rotational speed is increasing. The rotating shaft reaches the rated rotational speed range via this critical speed range. Furthermore, stationary parts may deform non-uniformly because of a temperature difference at start-up time, and thereby, the clearance may transitionally become the minimum due to differential thermal expansion in the vertical direction. In this way, if the fins and the opposing surface come into heavy contact when the clearance is the minimum at start-up time, there is a possibility that a vicious circle is repeated whereby the rotating shaft side is locally heated by the excessive heat generated by the sliding, causing bending of the shaft, which then causes even more heavy contact. In this regard, by applying the abradable material, the amount of heat generated by the sliding itself becomes small, and it is possible to achieve a design capable of allowing contact up to a certain degree.    Patent Citation 1: Japanese Unexamined Patent Application, Publication No. 2002-228013    Patent Citation 2: Japanese Unexamined Patent Application, Publication No. 2003-65076